The invention relates to a process of calibrating a gas metering instrument, in which a test chamber, before the introduction of the gas to be tested, is flushed with a flushing gas and subsequently filled with a calibrating gas having a predetermined composition of calibrating substance and carrier gas. Moreover, suitable equipment for implementing the process is described.
During the long term use of gas metering equipment which determines the gas composition in a test chamber, it is always necessary to carry out a calibration of the measuring arrangement as a whole at certain intervals of time. For this purpose, a test gas of fixed composition is conducted from a storage tank into the test chamber. However, before this can be done, it is necessary to flush the test chamber with a flushing gas, generally air, in order to flush out residual amounts of test gas, which may still be present, and to prevent distortion of the calibration value. To carry out the calibration, expensive pipelines and switching arrangements are required in order to supply metered amounts of (a) test gas, (b) flushing gas (inert gas) and (c) calibrating gas at the correct times. In addition, storage facilities for the premixed calibrating gas must be provided.
Such a process and equipment for implementing it is described in U.S. Pat. No. 3,792,272.
During the test cycle, a test chamber is filled over a test pipeline with the gas to be tested and the gas is analyzed in this test chamber. When a calibration of the test chamber is necessary, a flushing pipeline is connected, over suitable switching elements, to the test chamber. Through said flushing pipeline, ambient air is introduced as flushing gas into the test chamber. After the test chamber has been flushed, an additional calibrating gas pipeline is connected in a further step to the test chamber and the test chamber is disconnected from the pipeline carrying the flushing air, so that a calibrating gas of a particular composition flows through the calibrating gas pipeline into the test chmber. After the calibration is concluded, the calibrating gas pipeline is closed and the test chamber is connected to the test gas pipeline.
In the known process, it is a disadvantage that the calibrating gas tank thaaht has to be provided must contain a supply of premixed calibrating gas in large volume. For a calibration, not only the test chamber itself, but also the gas pipelines must be flushed. This increases the consumption of calibrating gas and a corresponding amount must be made available. The preparation of the calibration gas of predetermined composition is expensive and it must moreover be made available in a careful manner. For example, care must be taken to ensure that, while the gas metering instrument is being used, the composition of the calibrating gas does not change. For example, a component of the calibrating gas may condense or otherwise be deposited in the pipelines and may not reach the test chamber. Consequently, stabilizers must be added to the calibrating gas under certain conditions or care must be taken to ensure that the tanks containing the calibrating gas are exchanged from time to time.